Particle Characterization Using Electro-Acoustic Spectroscopy

Particle Characterization Using Electro-Acoustic Spectroscopy 

Richard W. O Brien, James K. Beattie, and Robert J. Hunter 

The Electrokinetic Sonic Amplitude (ESA) effect in this context refers to the generation of ultrasound by the application of an alternating electric field to a colloid. Previous reviews on the ESA have mainly focused on the determination of particle size and zeta potential from the ESA. While this is certainly a very important application of the ESA phenomenon, there is more information in the ESA spectmm than just particle size and zeta. It can be used, for instance, to determine the thickness of adsorbed polymer layers or the surface conductance under the shear plane. It is these other applications that will be our main interest here. To begin we will give an alternative explanation for the ESA phenomenon, one that allows a deeper understanding of the underlying physics. 

The ESA effect occurs because the particles are electrically charged. The applied field causes the particles to move back and forth, and this backwards and forwards motion generates the ultrasound. This is the usual explanation for the ESA effect. Sound is a backwards and forwards motion, so it seems reasonable that a backwards and forwards motion of the particles will generate sound, and indeed this is usually the case. But this simple explanation does not account for two important aspects of the ESA effect (i) the ESA effect does not occur in suspensions of neutrally buoyant particles, even though they move back and forth in the electric field, and (ii) the ESA sound waves are generated by the particles, and yet thqr appear to come from the electrodes. 

To simplify things we focus on the ESA generated by a colloid that lies between two parallel plate electrodes. To begin with, the suspension is attest and the electrodes are 

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Particle Characterization Using Electro-Acoustic Spectroscopy... 

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